There is always a need for more bandwidth in communications channels, to accommodate a larger number of users. The finite or limited availability of channel bandwidth, in turn, makes the efficient use of bandwidth an economic necessity. The transmission of speech signals over limited-bandwidth channels has been the subject of extensive investigation and improvement. These improvements have given rise to devices known in the art as vocoders. In general, vocoders include a transmitter which analyzes the voice signal to be transmitted, and extracts various characteristics of the speech. These characteristics are encoded in some fashion, and transmitted over the limited-bandwidth transmission channel to a vocoder receiver. The vocoder receiver receives the encoded signals, and reconstitutes the original voice signal.
The voice signals which are reconstituted by the vocoder receiver never include all of the information occurring in the original voice signal, because the bandwidth of the transmission channel is incapable of carrying all of the information in the original voice. Thus, the quality of the signal received at the output of a vocoder system depends in part upon the bandwidth of the channel over which the signal must be transmitted, and in part upon the efficiency or effectiveness with which the system analyzes and reconstitutes the voice within the available bandwidth.
Of necessity, there is a certain amount of distortion in transmission over a vocoder system, and this distortion is manifested as coding noise. Various schemes have been advanced for masking or reducing the amplitude or perceived amplitude of the coding noise, including the schemes described in U.S. patent applications filed on Jul. 13, 1998, Ser. No. 09/114,658 in the name of Grabb et al.; Ser. No. 09/114,661 in the name of Zinser et al. Ser. No. 09/114,662 in the name of Grabb et al.; Ser. No. 09/114,663 in the name of Zinser et al.; Ser. No. 09/114,664, in the name of Zinser et al.; and Ser. No. 09/114,659 in the name of Grabb et al. Related matter appears in copending applications Ser. Nos. 09/340,100, 09/340,101; and 09/340,102, filed Jun. 25, 1999 in the names of Ross et al; Van Stralen et al., and Ross et al., respectively. Among these schemes is one described in docket number RDMM25497, U.S. patent application Ser. No. 09/114,660, filed Jul. 13, 1998 in the name of Zinser et al., entitled SPEECH CODING SYSTEM AND METHOD INCLUDING VOICING CUT OFF FREQUENCY ANALYZER, in which the system gain is calculated using the root-mean-square (RMS) value of the linear predictive coding (LPC) residual according to ##EQU1## where r.sub.i are the residual samples, and N is the number of samples in a speech frame, which in one embodiment is 160 samples in a frame having duration of 20 msec. It was discovered that men's voices were not perceived as sounding as good as those of women after transmission of speech through the limited-bandwidth channel.
Improved vocoder arrangements are desired.